Surface mounted chip resistor with flexible leads

ABSTRACT

A chip resistor having first and second opposite ends includes a rigid insulated substrate having a top surface and an opposite bottom surface, a first electrically conductive termination pad and a second electrically conductive termination pad, both termination pads on the top surface of the rigid insulated substrate, a layer of resistive material between the first and second electrically conductive termination pads, and a first and a second flexible lead, each made of an electrically conductive metal with a solder enhancing coating. The first flexible lead attached and electrically connected to the first electrically conductive termination pad and the second flexible lead attached and electrically connected to the second electrically conductive termination pad. Each of the flexible leads has a plurality of lead sections facilitating bending around the end of the chip resistor.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.12/035,472, filed Feb. 22, 2008, which is incorporated by reference asif fully set forth.

FIELD OF THE INVENTION

The present invention relates to electronic passive components. Moreparticularly, the present invention relates to a Surface Mounted (SM)chip resistor having a flat resistive pattern between two conductingtermination pads. The invention also relates to a manufacturing methodfor transforming a surface mounted lead-less chip resistor into acomponent with flexible leads.

BACKGROUND OF THE INVENTION

SM resistors having flat resistive elements are produced in two mainconfigurations—lead-less, and with leads. In the first configuration,the resistors are lead-less rectangular chips with termination pads forelectrical connection at the two opposite ends of a resistive layer ontop of a ceramic substrate.

The first configuration of SM resistor may have different terminationconfigurations, including “Wrap Around Metallization” or “Flip Chip”configured termination. In chips with “Wrap Around Metallization”, thepads extend from the termination pads over the short sides of therectangular chip and to part of the chip's bottom. Soldering to a PCB isperformed according to one of these two configurations of theterminations. For chips having pads on top only, a “Flip Chip”configuration, with pads down. For chips with “Wrap AroundMetallization” the chip's metallized bottom is placed on the pads of thePCB.

The second configuration of a surface mount resistor is molded in aresin package with two flat leads attached to the termination pads,extending on two sides of the package, and bent to seat, for soldering,on the pads of a PCB.

The first configuration has the advantage of smaller dimensions andlower manufacturing costs, but has a limitation of failures of thesolder joint to the PCB's pads when subjected to high mechanical and/orthermal stresses, the latter due to a mismatch of Coefficients ofThermal Expansion (CTE) between the chip's ceramic substrate and thePCB's material. The probability of such failure increases with the sizeof the chip.

Such stresses occur when the ambient temperature changes and the chipdoes not expand or contract at same rate as the PCB, or when a loadapplied to the resistor causes a rise of its temperature while the PCBremains cooler, or when the PCB is flexed causing a change of thedistance between its pads. Thus, despite advances in the art, problemsremain.

What is needed is a solution to failures of chip resistors caused bymechanical and thermal stresses after the chips are soldered to aPrinted Circuit Board (PCB).

Therefore, it is a primary object, feature, or advantage of the presentinvention is to provide a low-cost method, suitable for mass production,of transforming lead-less chip resistors into devices with flexibleleads without significant increase of their size.

BRIEF SUMMARY OF THE INVENTION

According to one aspect of the present invention a chip resistor isprovided. The chip resistor includes a rigid insulated substrate havinga top surface and an opposite bottom surface. There is a firstelectrically conductive termination pad and a second electricallyconductive termination pad, both termination pads on the top surface ofthe rigid insulated substrate. There is also a layer of resistivematerial between the first and second electrically conductivetermination pads. A first and a second flexible lead, each made of anelectrically conductive metal is provided. The first flexible lead isattached and electrically connected to the first electrically conductivetermination pad and the second flexible lead is attached andelectrically connected to the second electrically conductive terminationpad. Each of the flexible leads if formed from a plurality of leadsections for facilitating bending of each of the flexible leads aroundone of the ends of the chip resistor Each of the flexible leads is bentaround one of the ends of the chip resistor.

According to another aspect of the present invention, a method formanufacturing a chip resistor with flexible leads from a leadless chipresistor is provided. The method includes providing a carrier strip witha lead-frame comprising a first and a second flexible lead, eachflexible lead made of an electrically conductive metal and comprised ofa plurality of lead sections, bending the first flexible lead and thesecond flexible lead around opposite ends of the leadless chip resistor,attaching the first flexible lead to a first termination pad of theleadless chip resistor, attaching the second flexible lead to a secondtermination pad of the leadless chip resistor, and separating theflexible leads from the carrier strip.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing of a first version of a lead-frame used formanufacturing of the chip resistors with flexible leads. The lead-frameconsists of a carrier strip with leads on its two sides.

FIG. 2 is a drawing of a second version consisting of a lead-frame splitinto two identical half lead-frames.

FIG. 3 shows a profile of the lead-frame per FIG. 2 after its sectionsare partially bent to form a nest for placing the resistor chip.

FIG. 4 is a top view of an assembled chip with the flexible leads.

FIG. 5 is view of the long side of the same.

FIG. 6 is view of the short side of the same.

FIG. 7 shows an area of a section of a flexible lead coated with resinto prevent coverage by solder during assembly onto a PCB.

FIG. 8 illustrates a perspective of one embodiment of an assembled chipwith flexible leads.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides for attaching lead-frames to lead-lesschip resistors in order to convert the lead-less chip resistors intochip resistors with flexible leads. The present invention provides adesign of lead-frames and an assembly method suitable for automated massproduction which enables the transformation of a finished lead-lessresistor chip into a high reliability resistor with flexible leads forSM assembly onto a PCB.

Multiple embodiments are shown including an embodiment where a chipresistors is placed on a lead-frame with the termination pads up as wellas an embodiment where the chip resistor is placed on a lead-frame withthe termination pads down.

In a first embodiment, the chip resistors are placed on a lead-framewith the termination pads up. FIG. 1 shows a lead-frame consisting of acentral carrier strip 2 having punched out areas 10 and the flexibleleads 4 on each side of the carrier strip 2. Each flexible lead 4consists of multiple sections such as a first section 12, a secondsection 14, and a third section 16. Two flaps 18 are attached onopposite ends of the third section 16.

A first groove 20 is etched or stamped to facilitate separation of theassembled chips from the carrier strip 2. In addition, grooves areetched or stamped to facilitate the bending of the lead sections 12, 14,16 around the resistor chip. A first groove 24 is between the first leadsection 12 and the second lead section 14. A second groove 26 is betweenthe second lead section 14 and a third lead section 16. Grooves 22 arealso present between the third section 16 and its flaps 18.

Assembly is performed by placing a flip chip on the lead-frame, with thechip's termination pads up, on two sections 12, applying a solder pasteon the pads and an adhesive such as glue to the flaps 18, bending theflaps 18 and sections 14 and section 16 to approach the chip's shortsides and termination pads, and reflowing the solder paste and curingthe adhesive or glue. The mechanical attachment of the flexible leads tothe chip, in addition to the solder joint, to the termination pad, isenhanced by cementing the flaps 18 to the chip.

Commonly used epoxy-glass PCB have a coefficient of thermal expansion(CTE) much higher than the CTE of a ceramic or similar substrate of theresistor chip. The mounting of chips onto the PCB requires a hightemperature for re-flowing the solder, and it is followed by coolingafter solder's solidification. As a result, the PCB contracts more thanthe chip introducing a thermal strain. The bottom section 16 of theflexible leads follows this movement of PCB's contraction thus avoidinga dangerous stress in the solder joint. Similarly, when the chipresistor heats up due to an applied load, it expands while the PCB isstill cold and again it causes a bending of the leads without stressingthe solder joint. The mechanical attachment of the flexible leads 4 tothe chip, in addition to the solder joint with the termination pad, isenhanced by the flaps 18 cemented to the chip. The flaps 18 take up ashear stress as a result of these thermal strains and stresses whichassures a high degree of the assembly's robustness.

FIG. 2 and FIG. 3 illustrate another embodiment. In the embodiment shownin FIG. 2 and FIG. 3, two opposite half lead-frames are used. FIG. 2 isa plan view. FIG. 3 is a profile, after partial bending. In thisembodiment flaps 18 and section 14 are first bent as shown in FIG. 3 toform a nest for placing the flip chips. Then, solder paste and glue areapplied to the nest. The chips are then placed face down with theirterminal pads on sections 16. After the reflowing the solder paste andcuring the glue the carrier strip of one half-frame can be bent alongthe grooves 20 and cut off to allow automated electrical measurement ofindividual chips and, if required, final trimming of their resistance toa very precise ohmic value.

An alternative method consists of using chips with gold plated pads, atin plated lead-frame and achieve the electrical interconnection byparallel-gap welding.

FIGS. 4, 5 and 6 show a plan view and profiles of an assembled chipresistor with flexible leads. A chip 30 includes termination pads 32,lead sections 12, 14 and 16 and flaps 18.

FIG. 7 shows a protective resin layer 14′ which is applied on both sidesof section 14 before assembly. The resin prevents, during soldering to aPCB, the creeping of the solder up the plates 18 which would reduce itsflexibility.

The process described above can also be applied to lead-less chipresistors with wrap-around metallization after these chips are covered,up to about half of their thickness, with a protective layer preventingthe solder to creep up the leads during the assembly on a PCB. This canbe achieved by dipping the chips to about half their thickness into aliquid thermosetting resin and curing the resin.

Thus, the present invention provides for transforming a leadless chipresistor into one with flexible leads. The lead-frames containing theflexible leads are designed in a way to enable alternative productionmethods, including reel to reel automated assembly for mass productionor the use of strips of lead-frame for hand processing of small batchesof chips.

Therefore various embodiments for providing a method of transforminglead-less chip resistors into devices with flexible leads have beenprovided. The present invention contemplates variations in the structureof the lead frame, the manner in which the lead frame is attached, andother variations within the spirit and scope of the invention.

1. A chip resistor having first and second opposite ends, comprising: arigid insulated substrate having a top surface and an opposite bottomsurface; a first electrically conductive termination pad and a secondelectrically conductive termination pad, both termination pads on thetop surface of the rigid insulated substrate; a layer of resistivematerial between the first and second electrically conductivetermination pads; a first and a second flexible lead, each made of anelectrically conductive metal with a solder enhancing coating; the firstflexible lead attached and electrically connected to the firstelectrically conductive termination pad and the second flexible leadattached and electrically connected to the second electricallyconductive termination pad; wherein each of the flexible leads beingcomprised of a plurality of lead sections for facilitating bending ofeach of the flexible leads around one of the ends of the chip resistorand each of the flexible leads being bent around one of the ends of thechip resistor.
 2. The chip resistor of claim 1 wherein the plurality ofsections includes a first lead section, a second lead section and athird section and wherein a first groove is between the first leadsection and the second lead section and wherein a second groove isbetween the second lead section and the third lead section.
 3. The chipresistor of claim 2 wherein the second lead section being coated in aresin
 4. The chip resistor of claim 2 further comprising first andsecond flaps extending from each first lead section.
 5. The chipresistor of claim 1 wherein the first and second flaps being bent downto and cemented to the substrate such that mechanical and thermalstresses in the leads create a shear stress in an interface of the flapsand substrate.
 6. The chip resistor of claim 1 wherein the each flexiblelead is bent around one of the ends of the chip resistor to form ahorizontal foot section suitable for soldering to a pad on a printedcircuit board (PCB).
 7. The chip resistor of claim 6 further comprisinga small gap between the flexible lead and the end of the substrate toallow contraction of the PCB when cooled after soldering the lead to thepad on the PCB.
 8. The chip resistor of claim 7 wherein at least one ofthe flexible lead sections includes a protective coating to avoidcreeping of liquid solder up the lead when the chip resistor is solderedto the PCB.
 9. The chip resistor of claim 1 further comprising a wraparound metallization for each of the termination pads and where a resincovers a portion of the wrap around metallization.
 10. A method formanufacturing a chip resistor with flexible leads from a leadless chipresistor, comprising: providing a carrier strip with a lead-framecomprising a first and a second flexible lead, each flexible lead madeof an electrically conductive metal and comprised of a plurality of leadsections; bending the first flexible lead and the second flexible leadaround opposite ends of the leadless chip resistor; attaching the firstflexible lead to a first termination pad of the leadless chip resistor;attaching the second flexible lead to a second termination pad of theleadless chip resistor; and separating the first flexible lead and thesecond flexible lead from the carrier strip.
 11. The method of claim 10wherein each of the flexible leads includes flaps and wherein the methodfurther comprises attaching the flaps to the leadless chip resistor. 12.The method of claim 11 wherein the step of attaching the flaps to theleadless chip resistor comprises cementing the flaps to the leadlesschip resistor.
 13. The method of claim 11 wherein the attaching thefirst flexible lead and the attaching the second flexible lead beingperformed using soldering.
 14. The method of claim 11 wherein theattaching the first flexible lead and the attaching the second flexiblelead being performed using welding.